Whiteflies are a common pest of plants, particularly greenhouse plants, and a vector of several plant diseases.
Despite their name, whiteflies are not true flies. Rather, they are in the order Hemiptera along with plant pests like aphids, mealybugs, and scales. There are multiple species of whitefly (family Aleyrodidae) that can damage ornamental plants and vegetable crops or become established in greenhouses. Adult whiteflies are tiny (1/16-1/10 in long) and their wings are covered in white wax. Wings are usually white, sometimes with gray markings. Nymphs differ greatly from the adult whitefly. The first nymphal stage, called a crawler, is barely visible and moves around the leaf before settling to begin feeding. Later instar nymphs may be visible on the underside of leaves. Late-stage nymphs are immobile and flattened with reduced legs, resembling small scale insects.
Nancy Gregory, University of Delaware, Bugwood.org.
Important Species: Bandwinged Whitely, Trialeurodes abutiloneus
Host Plants: very broad including cotton, cucurbits, and other vegetables.
David Cappaert, Bugwood.org
Greenhouse whitefly nymphs, black forms are parasitized.
Whitney Cranshaw, Colorado State University, Bugwood.org
Important Species: Greenhouse whiteflies, Trialeurodes vaporariorum
Host plants: very broad including most vegetables and herbaceous ornamentals. Also, may occur on avocado, fuchsia, gardenia, lantana and redbud.
W. Billen, Pflanzenbeschaustelle, Weil am Rhein, Bugwood.org.
Important Species: Sweet Potato Whitefly, Bemisia tabaci
Host Plants: very broad, including many herbaceous and some woody plants including cole crops, cotton, cucurbits, tomatoes, peppers, crape myrtle, lantana, roses, and hibiscus.
- Whiteflies are a common pest of many plants and can become especially injurious in greenhouse production.
- In summer, they can become established in the garden, affecting vegetables and flowers.
- They do not survive outdoors in areas of freezing winters, such as Colorado. Winter infestations may persist on indoor plants.
- Whiteflies can transmit plant pathogens and are resistant to many insecticides
- Colored sticky traps and certain biological controls are effective tactics in whitefly control.
Life History and Habits
Whiteflies are a common insect pest in greenhouses and on house plants. It can also become established on garden plants during the summer. The insect is a tropical/subtropical species, and it has a host range of more than 250 ornamental and vegetable plants. New infestations arise from the winged adult females moving to new plants. The female will lay eggs in a semicircular pattern while they feed. Nymphs emerge from the eggs in five to seven days and move a short distance before flattening themselves against the leaf to feed. All remaining immature stages are immobile. Under favorable conditions, a generation of whiteflies takes three to four weeks to complete. Each female can lay 400 eggs over a period of two months. Bemisia argentifolii is a new species that has been established in Colorado. It is endemic to southwestern US and Florida, where it causes severe damage to ornamentals and crops.
Whiteflies feed on phloem and heavy infestations cause a decline of plant vigor. Symptoms of injury include stunting, yellowing of foliage and premature leaf drop. They excrete excess sugars in a sticky waste product called honeydew which detracts from plant appearance and can allow sooty mold fungi to grow on the foliage. Whiteflies can have multiple generations in the field, and year-round infestations are possible indoors
Prevention is the best management. Infestations in the field and greenhouse frequently originate from infested plant materials. Carefully check all plants and quarantine them before moving them into a greenhouse with susceptible plants or planting in the field. Once whiteflies are established in a greenhouse, a host-free period can be an effective management strategy. This may mean removing susceptible plants for at least two weeks. Weed control in the greenhouses is an effective preventative tactic as well. Whiteflies are also attracted to yellow sticky traps, which can be used to monitor presence of the pest and are useful in early detection of whiteflies.
Small parasitic wasp, Encarsia formosa is an effective parasitoid of whiteflies. Larvae of these wasps develop within whitefly nymphs, which when parasitized turn black and die within several days. Minute pirate bugs can also be effective predators of whitefly nymphs, particularly in greenhouses.
Insecticidal control can be difficult since the insect is resistant to a wide range of insecticides. Repeated applications are usually required. Horticultural oils are useful, particularly on houseplants.